time: add timemono_since

time_mono()'s documentation references time_since(), which is nowhere
to be found.

A possible fix would be to just remove that stale reference.

Instead, this patch adds timemono_since(), which hopefully
captures the meaning of the original time_since().
Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/time/test/run-monotonic.c

@@ -7,7 +7,7 @@ int main(void)
 	struct timemono t1, t2;
 	struct timerel t3;
 
-	plan_tests(2);
+	plan_tests(3);
 
 	/* Test time_mono */
 	t1 = time_mono();
@@ -19,6 +19,7 @@ int main(void)
 	t3.ts.tv_nsec = 0;
 
 	ok1(time_less(timemono_between(t1, t2), t3));
+	ok1(time_less(timemono_since(t1), t3));
 
 	return exit_status();
 }

ccan/time/time.h

@@ -127,7 +127,7 @@ struct timeabs time_now(void);
  * This value is only really useful for measuring time intervals.
  *
  * See also:
- *	time_since()
+ *	timemono_since()
  */
 struct timemono time_mono(void);
 
@@ -320,6 +320,17 @@ static inline struct timerel timemono_between(struct timemono recent,
 	return t;
 }
 
+/**
+ * timemono_since - elapsed monotonic time since @old
+ * @old: a monotonic time from the past.
+ */
+static inline struct timerel timemono_since(struct timemono old)
+{
+	struct timemono now = time_mono();
+
+	return timemono_between(now, old);
+}
+
 /**
  * timeabs_sub - subtract a relative time from an absolute time
  * @abs: the absolute time.